• Built between 1173 and 1350, its height is nearly 56 metres.
• The outer diameter of the base is roughly 15.5 metres, with an inner diameter of about 7.5 metres. The wall thickness at the base is just over two metres.
• The total weight of the tower is 14,700 metric tons.
• From 1173—1250 the tower leaned to the north.
• From 1272 to the present day the tower has leaned to the south, from a roughly 1.5 metre lean at the end of the thirteenth century to a roughly 5.2 metre lean at the end of the twentieth century.

Monitoring the Movement

It was not until 1911 that there were instruments and methods available to track the changing lean of the tower accurately. The first surveys used a theodolite placed at a specific point to measure the angle between the first and seventh cornices.

In 1928, four ‘benchmarks’ were placed on the base of the tower. Their relative levels were used to deduce the lean of the tower.

In 1934 a pendulum was suspended inside the hollow interior of the tower and a highly accurate spirit level was placed in the instrument room at the first level. Although simple devices, they provided useful information.

In 1992 an electronic monitoring station was finally installed. Inclinometers automatically record and transmit the values of the north-south and east-west inclination at pre-determined intervals, which may be as little as four minutes. The instruments allow scientists not only to follow medium-term changes in inclination but also to monitor effects of wind and seismic activity.

Using Lead to Halt the Lean

In 1993 a counterweight of roughly 600 tonnes of lead ingots was placed on the north side of the base of the Tower to arrest the southward rotation. This led to the first recorded stabilisation of the monument in eight centuries of its history.

In 1995, the tower started to tilt again, and an additional 270 tonnes was added to halt the lean once again.

Recent Suggestions for Reducing the Lean of the Tower

Reducing the lean of the bell tower of the cathedral in Pisa poses an ongoing engineering challenge. Throughout history, attempts have had at best, no effect and at worst, made the tower tilt even more.

The earth removal strategy shown in the final stages of the programme seems to be having some success at reversing the tilt of the tower.

(See http://torre.duomo.pisa.it/towersposters/english_version/ and select ‘current interventions’ in site selection.)

An alternative suggestion put to the committee involved a slightly different three-stage approach.

1. Attaching steel support cables, 90 metres long and 9 centimetres thick, to the tower. The cables would attach into a cement ring sunk into the ground 90 metres away from the tower, gently supporting it.

2. Using thin wires covered in chipped diamond as long, thin saws running parallel to each other under the base of the tower. The saws would carve out soil from beneath the north side of the tower. The silt would be slowly removed by bringing the wires out repeatedly and drawing bits of soil with them. As the ground is taken from under the north side of the tower the cables will tighten, settling the tower upright.

3. Build a cement base under the tower, preventing it from shifting in the future.

Two less technical suggestions have also been put forward, perhaps not entirely seriously!

(a) Attaching a huge helium balloon to the south side of the tower.
(b) Landscaping the surrounding area to make the tower appear straight.